1. Field of the Invention
The present invention relates to a semiconductor device having a boost DC—DC converter for converting input electric power to output electric power having a voltage higher than that of the input electric power. In particular, the present invention relates to a semiconductor device having a boost DC—DC converter that operates at the output voltage when actuated.
2. Description of the Related Art
FIG. 3 illustrates a semiconductor device having a conventional boost DC—DC converter.
As illustrated in FIG. 3, the semiconductor device includes: a power supply 101; a booster circuit 102 for converting electric power supplied by the power supply 101 to electric power having higher voltage; a Schottky diode 304; and a load 103 to be operated by the boosted electric power. The Schottky diode 304 is provided between an input terminal 111 of the booster circuit 102 and a power supply terminal 112 of the booster circuit 102 such that a direction from the input terminal 111 of the booster circuit 102 to the power supply terminal 112 of the booster circuit 102 is a forward direction. Further, an output terminal 113 of the booster circuit 102 is connected to the power supply terminal 112 of the booster circuit 102 and to the load 103.
With the above-described structure, in starting the booster circuit 102, the power supply 101 supplies electric power to the input terminal 111 of the booster circuit 102 and the electric power is further inputted to the power supply terminal 112 of the booster circuit 102 via the Schottky diode 304, to thereby bring the booster circuit 102 into operation. Once the booster circuit 102 is actuated and the boosted electric power is generated at the output terminal 113 of the booster circuit 102, the booster circuit 102 returns the boosted electric power to the power supply terminal 112 of the booster circuit 102 to thereby maintain its boosting operation. It is generally assumed that the boosted electric power would not flow back to the power supply 101 by the rectifying action of the Schottky diode 304. However, as described above, in starting the booster circuit 102, the voltage of the power supply 101 is supplied to the power supply terminal 112 of the booster circuit 102 via the Schottky diode 304, and therefore the booster circuit 102 cannot be brought into operation unless the power supply voltage that is higher than the lowest actuation voltage of the booster circuit 102 by a forward drop voltage of the Schottky diode 304 (hereinafter briefly referred to as Vf) is inputted. Further, when the load 103 is being operated upon start-up of the booster circuit 102, Vf is further increased. In addition, when parasitic resistance or the like is connected in series between the input terminal 111 of the booster circuit 102 and the power supply terminal 112 of the booster circuit 102, it is necessary to input power supply of still higher voltage to actuate the booster circuit 102.
Therefore, in a conventional boost DC—DC converter disclosed in JP 05-304765 A, a switching device is provided between the output terminal 113 of the booster circuit 102 and the load 103 of the boost DC—DC converter of the above structure, and upon starting the booster circuit 102, the switching device is turned off to prevent an operation of the load 103 from increasing Vf, thereby making the power supply voltage for actuating the booster circuit 102 as low as possible.
As described above, in starting a conventional boost DC—DC converter, there is a problem in that the conventional boost DC—DC converter cannot be actuated unless the input voltage of power supply that is higher than the lowest voltage at which the internal booster circuit can be actuated by Vf of the Schottky diode is inputted.